Re: [RFC/PATCH v3 04/16] s390/mm: add gmap PMD invalidation notification

[David Hildenbrand <david@redhat.com>]

On 13.02.2018 15:54, Janosch Frank wrote:
> On 13.02.2018 15:36, David Hildenbrand wrote:
>> On 09.02.2018 10:34, Janosch Frank wrote:
>>> For later migration of huge pages we want to write-protect guest
>>> PMDs. While doing this, we have to make absolutely sure, that the
>>> guest's lowcore is always accessible when the VCPU is running. With
>>> PTEs, this is solved by marking the PGSTEs of the lowcore pages with
>>> the invalidation notification bit and kicking the guest out of the SIE
>>> via a notifier function if we need to invalidate such a page.
>>>
>>> With PMDs we do not have PGSTEs or some other bits we could use in the
>>> host PMD. Instead we pick one of the free bits in the gmap PMD. Every
>>> time a host pmd will be invalidated, we will check if the respective
>>> gmap PMD has the bit set and in that case fire up the notifier.
>>>
>>> In the first step we only support setting the invalidation bit, but we
>>> do not support restricting access of guest pmds. It will follow
>>> shortly.
>>>
>>> Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
>>
>> [...]
>>
>>> +
>>> +/**
>>> + * gmap_pmd_split - Split a huge gmap pmd and use a page table instead
>>> + * @gmap: pointer to guest mapping meta data structure
>>> + * @gaddr: virtual address in the guest address space
>>> + * @pmdp: pointer to the pmd that will be split
>>> + *
>>> + * When splitting gmap pmds, we have to make the resulting page table
>>> + * look like it's a normal one to be able to use the common pte
>>> + * handling functions. Also we need to track these new tables as they
>>> + * aren't tracked anywhere else.
>>> + */
>>> +static int gmap_pmd_split(struct gmap *gmap, unsigned long gaddr, pmd_t *pmdp)
>>> +{
>>> +	unsigned long *table;
>>> +	struct page *page;
>>> +	pmd_t new;
>>> +	int i;
>>> +
>>
>> That's interesting, because the SIE can now suddenly work on these
>> PGSTEs, e.g. not leading to intercepts on certain events (like setting
>> storage keys).
>>
>> How is that intended to be handled? I assume we would somehow have to
>> forbid the SIE from making use of the PGSTE. But that involves clearing
>> certain interception controls, which might be problematic.
> 
> Well, cmma is disabled and storage keys should only be a problem, when
> the pte is invalid without the pgste lock, which should never be the
> case for split pmds.
> 

Are you sure? Because the SIE would suddenly stark working on guest
storage keys stored in the PGSTE if I am not mistaking? So I would
assume that there would have to be some kind of a sync.

But I don't have any documentation at hand, so i can't tell :)

-- 

Thanks,

David / dhildenb